J. W. Bottenheim

Biogenic hydrocarbons in the atmospheric boundary layer: A review

Nonmethane hydrocarbons are ubiquitous trace atmospheric constituents yet they control the oxidation capacity of the atmosphere. Both anthropogenic and biogenic processes contribute to the release of hydrocarbons to the atmosphere. In this manuscript, the state of the science concerning biosynthesis, transport, and chemical transformation of hydrocarbons emitted by the terrestrial biosphere is reviewed. In particular, the focus is on isoprene, monoterpenes, and oxygen-ated hydrocarbons. The generated science during the last 10 years is reviewed to explain and quantify hydrocarbon emissions from vegetation and to discern impacts of biogenic hydrocarbons on local and regional atmospheric chemistry. Furthermore, the physiological and environmental processes controlling biosynthesis and production of hydrocarbon compounds are reported on. Many advances have been made on measurement and modeling approaches developed to quantify hydrocarbon emissions from leaves and forest ecosystems. A synthesis of the atmospheric chemistry of biogenic hydrocarbons and their role in the formation of oxidants and aerosols is presented. The integration of biogenic hydrocarbon kinetics and atmospheric physics into mathematical modeling systems is examined to assess the contribution of biogenic hydrocarbons to the formation of oxidants and aerosols, thereby allowing us to study their impacts on the earths climate system and to develop strategies to reduce oxidant precursors in affected regions.

Correlation of ozone with NOy in photochemically aged air

During the summer of 1988, measurements of photochemical trace species were made at a coordinated network of seven rural sites in the eastern United States and Canada. At six of these sites concurrent measurements of ozone and the sum of the reactive nitrogen species, NOy, were made, and at four of the sites a measure for the reaction products of the NOx oxidation was obtained. Common to all sites, ozone, in photochemically aged air during the summer, shows an increase with increasing NOy levels, from a background value of 30–40 parts per billion by volume (ppbv) at NOy mixing ratios below 1 ppbv to values between 70 to 100 ppbv at NOy levels of 10 ppbv. Ozone correlates even more closely with the products of the NOx oxidation. The correlations from the different sites agree closely at mixing ratios of the oxidation products below 5 ppbv, but systematic differences appear at higher levels. Variations in the biogenic hydrocarbon emissions may explain these differences.

Measurements of C2-C6hydrocarbons during the Polar Sunrise1992 Experiment: Evidence for Cl atom and Br atom chemistry

The authors report the results of grab samples made in the Arctic winter atmosphere, which were analyzed for the presence of nonmethane hydrocarbons (C{sub 2} - C{sub 6}). The normal removal reactions for such species are due to OH radical reactions, which are expected to be strongly suppressed in winter months. The nonmethane hydrocarbons were observed to correlate well with methane during most of the winter. In April, during low ozone periods, additional depletions of acetylene were observed, which could be correlatied at least in part with Cl concentrations. Not all the variations could be accounted for due to chlorine reactions, and the authors argue that this could be the result of bromine reactions, whose presence would also correlate well with the observed ozone depletions.

Snowpack photochemical production of HONO : a major source of OH in the Arctic boundary layer in springtime

Both snow manipulation experiments and ambient measurements during the Polar Sunrise Experiment 2000 at Alert (Alert2000) indicate intensive photochemical production of nitrous acid (HONO) in the snowpack. This process constitutes a major HONO source for the overlying atmospheric boundary layer in the Arctic during the springtime, and sustained concentrations of HONO high enough that upon photolysis they became the dominant hydroxyl radical (OH) source. This implies a much greater role for OH radicals in Arctic polar sunrise chemistry than previously believed. Although the observations were made in the high Arctic, this finding has a significant implication for the boundary layer atmospheric chemistry in Antarctica during sunlit seasons and in the mid to high latitudes of the Northern Hemisphere during the winter and spring seasons when approximately 50% of the land mass may be covered by snow.

Anthropogenic aerosols and gases in the lower troposphere at Alert, Canada in April 1986

During April 1986, as part of an international arctic air chemistry study (AGASP-2), ground level observations of aerosol trace elements, oxides of sulphur and nitrogen and particle number size distribution were made at Alert Canada (82.5N, 62.3W). Pollution haze was evident as indicated by daily aerosol number (size > 0.15 μm diameter) and SO4= concentrations in the range 125 – 260 cm−3 and 1.6 – 4.5 μg m−3, respectively. Haze and associated acidic gases tended to increase throughout the period. SO2 and peroxyacetylnitrate (PAN) mixing ratios were in the range 140 – 480 and 370 – 590 ppt(v), respectively. About 88% of the total end-product nitrogen was in the form of PAN. In air dried to 2% relative humidity by warming to room temperature, the aerosol mass size distribution had a major mode at 0.3 μm diameter and a minor one at 2.5 μm. Aerosol mass below 1.5 μm was well correlated with SO4=, K+ and PAN. There was a steady increase in the oxidized fraction of total airborne sulphur and nitrogen oxide throughout April as the sun rose above the horizon and remained above. The mean oxidation rate of SO2 between Eurasia and Alert was estimated as 0.25 – 0.5% h−1. The molar ratio of total nitrogen oxide to total sulphur oxide in the arctic atmosphere (0.67±0.17) was comparable to that in European emissions. A remarkably strong inverse correlation of filterable Br and O3 led to the conclusion that O3 destruction and filterable Br production below the Arctic surface radiation inversion is associated with tropospheric photochemical reactions involving naturally occurring gaseous bromine compounds.

Tracing the Origin and Fate of NOx in the Arctic Atmosphere Using Stable Isotopes in Nitrate

Atmospheric nitrogen oxides (NOx =NO+ NO2) play a pivotal role in the cycling of reactive nitrogen (ultimately deposited as nitrate) and the oxidative capacity of the atmosphere. Combined measurements of nitrogen and oxygen stable isotope ratios of nitrate collected in the Arctic atmosphere were used to infer the origin and fate of NOx and nitrate on a seasonal basis. In spring, photochemically driven emissions of reactive nitrogen from the snowpack into the atmosphere make local oxidation of NOx by bromine oxide the major contributor to the nitrate budget. The comprehensive isotopic composition of nitrate provides strong constraints on the relative importance of the key atmospheric oxidants in the present atmosphere, with the potential for extension into the past using ice cores.

A computer model study of multiphase chemistry in the Arctic boundary layer during polar sunrise

A multiphase chemical box model of Arctic halogen chemistry has been developed using a PC-based modeling program developed by Environment Canada called the Chemical Reactions Modeling System (CREAMS). The multiphase model contains 125 gas phase reactions, 19 photolysis reactions, and 16 aqueous reactions occurring in suspended aerosol particles and the quasi-liquid component of snow. The model simulates mass transfer of species between the gas phase and particles, and between the gas phase and the snowpack. Model simulations were conducted for the Arctic for the period April 16 to April 24 at 245 K within a 400 m boundary layer. The complete model simulates halogen-catalyzed ozone depletion within 5 days from the start of the model run, via known gas and heterogeneous phase activation mechanisms. A critically important model reaction is BrO + HCHO → HOBr + CHO, which has a substantial impact on gas phase HOBr, and subsequent condensed phase chemistry. When coupled with a necessary snowpack efflux of aldehydes, required to maintain the aldehyde concentrations at observed levels, the new BrO chemistry has a significant impact on the concentrations of gas phase bromine species, particle bromide, and chlorine atoms, through chemistry occurring in the snowpack. We also find that O3 depletion cannot be simulated without the presence of heterogeneous halogen chemistry occurring in the snowpack and that the rate of O3 depletion is limited by the mass transfer rate of HOBr to the snowpack.

Use of backward trajectories to interpret the 5‐year record of PAN and O3 ambient air concentrations at Kejimkujik National Park, Nova Scotia

Air parcel trajectory data are used in two ways to elucidate the temporal trend analysis of 5 years of peroxyacetyl nitrate (PAN) and O3 observations at a rural site in eastern Canada. In the first method, “probability of residence” contours are constructed to determine the most probable origin of air parcels containing the highest and lowest 10% of PAN and O3 mixing ratios. High PAN is found to emanate always from areas of high anthropogenic activity, except when the transport path is to a large extent over the ocean, especially in the summer. High O3 originates from the same regions except in the winter when because of low photochemical activity, O3 is actually titrated and air from less populated areas is richer in O3. The trajectories indicate that transport at higher altitude leads to higher mixing ratios; this is especially the case for O3 in winter and spring. The complementary method of clustering the trajectories has allowed qualitative derivation of seasonal cycles for background and polluted air masses of different origin. Background air from the north shows a distinct PAN maximum in March; it is discussed that this could be either due to enhanced photochemistry or to the import of polluted air from the Arctic. Polluted air masses show the same March peak, but a second peak in late summer/early fall. Oceanic air from the south has a January maximum in PAN but otherwise is consistently low in PAN. O3 also has a spring maximum, but in polluted air it is broader stretching into the summer. It is postulated that this is due to additional O3 formation in the summer, while in the winter, actual O3 loss is indicated for polluted air. By inference it is deduced that the second PAN peak in polluted air is also due to additional formation in comparison with background air, while in the summer, extra PAN loss mechanisms operate that are less important for O3. Indications for both dry deposition and thermal decomposition have been found.

A case study of gas-to-particle conversion in an eastern Canadian forest

Aerosol and trace gas measurements were made at Kejimkujik National Park, Nova Scotia, Canada, during the summer of 1996. A case study from July 7-8 provides evidence of nucleation and condensation of products related to the oxidation of different biogenic emissions. Particles from 5 nm to 50 nm in diameter evolved during the afternoon and early evening associated with variations in isoprene. Late in the evening the α- and β-pinene mixing ratios and the aerosol particle volume increased. Soon after, there was a sharp increase in RO 2 H/H 2 O 2 that persisted until about 0100 LT. The initial increases in the pinenes and aerosols were strong and influenced by changes in winds. After 2200 LT, and into the early morning, the winds were relatively steady, and the α-and β-pinene mixing ratios continually decreased. The decay of α-pinene is explained through reaction with O 3 . However, the addition of OH radicals from the reaction of terpenes with O 3 is necessary to explain the observed rate of decay of β-pinene. During the same time, the aerosol volume increased with the decrease in α- and β-pinene. The volume increase was distributed 40:60 between particles in a mode centered at 80-90 nm and particles > 150 nm. The fine particle mass concentrations of the measured inorganic ions (sulfate, nitrate, chloride, ammonium, sodium, and calcium) and organic ions (oxalate, formate, acetate, pyruvate, propionate) account for 25-30% of the total aerosol volume during the period (2.7 μm 3 cm -3 ) indicating that the aerosol volume increase was due to unidentified species. Assuming that the increase in the aerosol was the result of products from the oxidation of α- and β-pinene, an aerosol mass yield of 13% is estimated. The concentrations of cloud condensation nuclei active at 0.2% supersaturation were enhanced by the appearance of the 80-90 nm mode pointing to at least some of these forest-generated particles as being able to serve as nuclei for cloud droplets at common atmospheric supersaturations.

Relationships between organic nitrates and surface ozone destruction during Polar Sunrise Experiment 1992

Concurrent measurements of total reactive odd nitrogen species (i.e., NOy) and its major components, including organic nitrates, were carried out during 1992 Polar Sunrise Experiment (PSE92) at Alert, Northwest Territories, Canada, to investigate the episodic depletion of surface level ozone following polar sunrise. A series of C3-C7 alkyl nitrates formed from the atmospheric oxidation of hydrocarbons was measured daily during the 13-week study period (January 22 to April 22). In addition, a large number of gas chromatography/electron capture detector (GC/ECD) peaks with retention times greater than those of the hexyl nitrates were also identified as species containing −ONO2 group(s), using a nitrogen specific detector. The total concentrations of these organic nitrates ranged from 34 to 128 parts per trillion by volume and the distribution in the dark period was found to be similar to that found for rural lower-latitude air masses. In contrast to observations made at lower latitudes where alkyl nitrates make a relatively small contribution to NOy, the organic nitrates at Alert were found to contribute between 7 and 20% of the total odd nitrogen species. After polar sunrise the total concentrations of these organic nitrates decreased steadily, due primarily to the consumption of larger (>C4) alkyl nitrates. The C3 alkyl nitrate concentrations showed little variation during this study. During ozone depletion episodes in April there was a positive correlation between the concentration of the larger organic nitrates and ozone. Most surprisingly, the ratio of concentrations of isomeric alkyl nitrates with carbon numbers ≥5, and in particular those involving the C5 isomers, was found to show substantial variations coinciding with the O3 depletion events. This change in the isomeric alkyl nitrate ratios implies a substantial chemical processing of the air masses exhibiting ozone depletion. The possible mechanisms, which must involve consumption of the organic nitrates by either OH radicals or Cl atoms, are discussed in the context of the chemical and meteorological observations conducted at Alert during these ozone depletion events.